We Are All Microbes

Microcosmos: Four Billion Years of Microbial Evolution, co-authored by Lynn Margulis and her son Dorion Sagan, was first published twenty years ago. Astrobiology Magazine recently interviewed Margulis, to find out how her and Sagan’s ideas have stood the test of time. In this, the second part of a four-part interview, she talks about four specific microbial organisms that, through fusion, yielded modern plant and animal cells.

Fluorescence micrographs of cyanobacteria. About 2 billion years ago, cyanobacteria – oxygen-producing photosynthetic prokaryotes that used to be called blue-green algae – were responsible for launching the process that increased the concentration of atmospheric oxygen from less than 1 percent to about 20 percent today, making possible the evolution of humans and other animals. Credit: Mary Sarcina University College London

Astrobiology Magazine: In Microcosmos, you detailed four specific microorganisms that you thought were involved, through symbiogenesis, in the creation of various eukaryotic cells, the type of cells that animals and plants are made of. At the time, those ideas were not well accepted. Has that changed?

Lynn Margulis: Well, we’ve won three out of four.

Nobody today doubts that chloroplasts began as cyanobacteria. Chloroplasts are the little green dots in the cells of plants and algae, in which all photosynthesis occurs. Photosynthesis, the conversion of sunlight as energy to food and cell material, is fundamentally a bacterial virtuosity. It began in a specific group of oxygen-producing photosynthetic bacteria that, by definition, are cyanobacteria. If they’re green, they’re photosynthetic. They make food only in the sunlight, because they require sunlight for their source of energy. They take carbon dioxide out of the atmosphere, and fix it, that is, chemically change it to food and body, and they produce oxygen as waste. That series of changes is done by cyanobacteria exclusively. They’re the only organisms that can make the oxygen and make the food that everything else needs.

Well, you say, can’t plants do that? And the answer is yes, but plants are something that hold up cyanobacteria. That’s all plants are. It’s the cyanobacteria in the plants that do that transformation. You say, well, can’t algae in the water, green water scum, can’t they do it? And the answer is, yes, but the algae are something that brings the little green things inside the scum to the light. So the answer is: nothing but cyanobacteria can make our food and produce our oxygen.

We like to call them the greater bacteria, or the greatest bacteria, because they are. And they’re in only three forms: they’re in cyanobacteria (what used to be called blue-green algae) all by themselves; or they’re in algae; or they’re in plants. But fundamentally, if you cut them out of the plant cell, and throw away the rest of the plant cell, the little green dot is the only thing that can do that oxygen production. That is the greatest achievement of life on Earth, and it occurred extremely early in the history of life. Who knows whether it’s 3 billion years ago, or 2.7 billion, or 3.5 billion, but it’s that kind of time. And the idea that those little organelles, those little bodies inside of cells, started as free-living cyanobacteria is completely accepted by everybody who even thinks about these problems.

So that’s one out of four.

AM: Number two is mitochondria, right?

Is the image astronomical or biological, as small as 100 microns or as large as 100’s of millions of miles? Answer: Biological, Chromosomes stained green; Mitochondria red; cytoskeleton blue.

LM: Yes. Mitochondria are little dots inside the cells of animals and plants and fungi, and all sorts of other organisms with hard names–those mitochondria are where the oxygen is actually respired. Mammals, including people, take oxygen out of the air into the bloodstream, and carry the oxygen all over the body, to all the cells. Inside each cell, the oxygen reacts with hydrogen atoms that are stripped off food as the food is converted into body parts and used for energy. This oxidation of the foodstuffs is carried out by little particles, the mitochondria, that are one micron, the size of bacteria, inside the animal cell, inside the plant cell, inside the amoeba cell, inside the mushroom cell, and so on. That’s what respiration is, and that respiration comes from bacteria that used to be free-living. We know a lot about those bacteria. They used to be able to swim, they used to be able to break down glucose all by themselves, etc. The idea is that mitochondria are from bacteria was harder to accept, but it’s now acceptable.

So that’s two out of four.

AM: Okay, and number three?

LM: Well, then the question is, What type of cell incorporated the mitochondria, and eventually the chloroplasts, some of them, into itself? What was the original eukaryotic cell? The original cell was an archaebacterium. It was sulfanogenic; it made hydrogen sulfide. We have just reviewed the evidence for that. People are not against that idea at all, because there’s a lot of molecular biological evidence for that.

So that’s three out of four.

AM: And what’s the fourth one? What’s left?

LM: The problem I’m still wrestling with is the origin of cilia, which are exactly the same in cross-section as sperm tails. That’s the piece that’s not been proven, the origin of the wiggly things. They all have strikingly identical 9-fold symmetry in cross-section, so it makes them easily identifiable. We believe the origin of that cell structure is from a free-living organism called a spirochete. That’s the part that has been rejected, based on the usual nothing – based on prejudice.

If you look at Microcosmos, you’ll see what we call spirochetal secret agents. But it’s harder to put your mind around. You don’t often hear of this connection between free-living bacteria and the movement inside cells, because there are so many different names associated with these motile structures. Historically they were approached by such different people in so many different studies in so many different fields and so many different countries.

Spirochetes are infamous because they are know to be the infectious agents of both syphilis and Lyme disease, and periodontal diseases are associated with oral spirochetes. Four of the spirochetes have been sequenced, because they’re of medical interest.

But a colleague (through the literature only – we don’t know her) named Galena Dubinina, a senior-level microbiologist at Moscow University, has sequenced the relevant spirochetes, the ones that can be directly compared with what we’re claiming grew into being the cilia. And so what we are working on is either confirming or negating our predictions about the free-living version. We have these organelles, the cilia, on the cells. We now have, because of her work, the right spirochetes to study the sequences in, and we have very explicit predictions. We’re trying to do that comparison, which is exactly what was done to prove the mitochondrial ancestry from the alpha-Proteobacteria, and the chloroplast ancestry from the Cyanobacteria.

Those same types of arguments are now, for the first time, usable to prove the spirochete origin of cilia. So that’s what we want to do, confirm that last, fourth prediction. We want to win four out of four.